The proposed studies focus on the clinical, epidemiologic, and virologic characterization of primary HHV-6 infection and the possible neurotropism of some HHV-6 strains. Preliminary studies at Rochester suggest that HHV-6 is a major cause of morbidity and healthcare costs for children in the first two years of life, accounting for 11% of Emergency Department visits for <2 year olds and 15-20% for 6-12 month olds. The proposed studies aim to delineate the importance of HHV-6 infection in childhood by describing the epidemiology, clinical, and laboratory course associated with primary infection. The source and means of transmission of HHV-6 to the infant and whether HHV-6 persists of reactivates after primary infection in children will be determined. The extent of genetic variability among strains of HHV-6 from such children and family members will be defined. These studies will further seek to determine if neurotropic strains exist and if the human central nervous system is a site of HHV-6 persistence. Children <2 years of age presenting to the Emergency Department with acute febrile illnesses will be enrolled and examined (n=1500-1800). Blood, respiratory secretions and, when possible, cerebrospinal fluid (CSF) will be obtained for HHV-6 isolation, identification by polymerase chain reaction (PCR), and serology. An estimated 150-180 children with primary viremic HHV-6 infection will thus be identified and followed over the subsequent year to determine the clinical course and sequelae of primary HHV-6 infection and will have repeated laboratory studies to determine whether HHV-6 persists and, if so, the sites of persistent infection. Children with primary HHV-6 infection will be compared to matched children with acute illness and negative HHV-6 cultures in order to determine the distinctive epidemiology, demographics, clinical and laboratory characteristics associated with HHV-6 infection. The source and means of transmission of HHV-6 to infants with primary infection will be delineated by the study of 1) 20 pairs of mothers and their HHV-6 positive infants and 2) newborn siblings of HHV-6 positive children. These children will have long-term follow-up with serial samples of blood and respiratory secretions. Isolates of infants and family members will be compared. Genetic and phenotypic variability among and within strains of HHV-6 will be defined by identifying variable domains of the HHV-6 genome which will be used as "molecular fingerprints" by two methods 1) sequence analysis of PCR-amplified DNA fragments and 2) restriction endonuclease digestive profiles of virion DNA. Whether HHV-6 strains can be divided into two groups based on antigenicity and host cell tropism will also be assessed by strain reactivity with a panel of monoclonal antibodies and by growth in various primary and continuous cell lines. Neurotropism of HHV-6 will be studied by detection of HHV-6 and/or antibody in the CSFs of children with primary of past HHV-6 infection and by PCR detection in post-mortem brain tissue of children dying of various causes, including AIDS. Neurotropic strains will be examined by using PCR to amplify directly HHV-6 gene sequences and strains compared by restriction endonuclease digestion profiles and by selective cell tropism in primary fetal neural cell types.